mago nashi mediates the posterior follicle cell-to-oocyte signal to organize axis formation in Drosophila

How introns influence and enhance eukaryotic gene expression

Although it has been known since the late 1970s that intron-containing and intronless versions of otherwise identical genes can exhibit dramatically different expression profiles, the underlying molecular mechanisms have only lately come to light. This review summarizes recent progress in our understanding of how introns and the act of their removal by the spliceosome can influence and enhance almost every step of mRNA metabolism. A rudimentary understanding of these effects can prove invaluable to researchers interested in optimizing transgene expression in eukaryotic systems.

Ovarian differentiation and human embryo quality. 1. Molecular and morphogenetic homologies between oocytes and embryos in Drosophila, C. elegans, Xenopus and mammals

Knowledge on the formation of oocytes and follicles in Drosophila, C. elegans and Xenopus, and the genetic regulation of polarities and embryo growth, has been related to comparable data in mammalian oocytes and embryos. Initially, details of the nature of the regulatory processes in the non-mammals are described, with considerable attention being paid to the role of individual genes and their specific functions. The molecular genetic aspects of these developmental processes are discussed in detail. Attention then turns to mammals, to identify, describe and evaluate their homologies with the lower animals and flies. Several of these homologies are described, including genes regulating primary ovarian failure and various aspects of early embryonic growth. The polarized distribution of genes in mammalian oocytes and embyros is discussed, together with the implications in the form of differentiation in the early embryo. Morphogenetic systems operative during follicle maturation, fertilization and cleavage are described and related to similar processes in lower forms. These events include ooplasmic and pronuclear rotations, the form of ooplasmic inheritance in early blastomeres and the establishment of embryonic axes. Models of early mammalian development are considered.

Molecular characterization of mouse gastric epithelial progenitor cells

The adult mouse gastric epithelium undergoes continuous renewal in discrete anatomic units. Lineage tracing studies have previously disclosed the morphologic features of gastric epithelial lineage progenitors (GEPs), including those of the presumptive multipotent stem cell. However, their molecular features have not been defined. Here, we present the results of an analysis of genes and pathways expressed in these cells. One hundred forty-seven transcripts enriched in GEPs were identified using an approach that did not require physical disruption of the stem cell niche. Real-time quantitative RT-PCR studies of laser capture microdissected cells retrieved from this niche confirmed enriched expression of a selected set of genes from the GEP list. An algorithm that allows quantitative comparisons of the functional relatedness of automatically annotated expression profiles showed that the GEP profile is similar to a dataset of genes that defines mouse hematopoietic stem cells, and distinct from the profiles of two differentiated GEP descendant lineages (parietal and zymogenic cell). Overall, our analysis revealed that growth factor response pathways are prominent in GEPs, with insulin-like growth factor appearing to play a key role. A substantial fraction of GEP transcripts encode products required for mRNA processing and cytoplasmic localization, including numerous homologs of Drosophila genes (e.g., Y14, staufen, mago nashi) needed for axis formation during oogenesis. mRNA targeting proteins may help these epithelial progenitors establish differential communications with neighboring cells in their niche.

Messenger-RNA-binding proteins and the messages they carry

From sites of transcription in the nucleus to the outreaches of the cytoplasm, messenger RNAs are associated with RNA-binding proteins. These proteins influence pre-mRNA processing as well as the transport, localization, translation and stability of mRNAs. Recent discoveries have shown that one group of these proteins marks exon exon junctions and has a role in mRNA export. These proteins communicate crucial information to the translation machinery for the surveillance of nonsense mutations and for mRNA localization and translation.

RNA processing: splicing and the cytoplasmic localisation of mRNA

An unexpected link has been discovered between pre-mRNA splicing in the nucleus and mRNA localisation in the cytoplasm. The new findings suggest that recruitment of the Mago Nashi and Y14 proteins upon splicing of oskar mRNA is an essential step in the localisation of the RNA to the posterior pole of the Drosophila oocyte.

Getting the message across: the intracellular localization of mRNAs in higher eukaryotes

The intracellular localization of mRNA, a common mechanism for targeting proteins to specific regions of the cell, probably occurs in most if not all polarized cell types. Many of the best characterized localized mRNAs are found in oocytes and early embryos, where they function as localized determinants that control axis formation and the development of the germline. However, mRNA localization has also been shown to play an important role in somatic cells, such as neurons, where it may be involved in learning and memory. mRNAs can be localized by a variety of mechanisms including local protection from degradation, diffusion to a localized anchor, and active transport, and we consider the evidence for each of these processes, before discussing the cis-acting elements that direct the localization of specific mRNAs and the trans-acting factors that bind them.

Translational regulation and RNA localization in Drosophila oocytes and embryos

Translational control is a prevalent means of gene regulation during Drosophila oogenesis and embryogenesis. Multiple maternal mRNAs are localized within the oocyte, and this localization is often coupled to their translational regulation. Subsequently, translational control allows maternally deposited mRNAs to direct the early stages of embryonic development. In this review we outline some general mechanisms of translational regulation and mRNA localization that have been uncovered in various model systems. Then we focus on the posttranscriptional regulation of four maternal transcripts in Drosophila that are localized during oogenesis and are critical for embryonic patterning: bicoid (bcd), nanos (nos), oskar (osk), and gurken (grk). Cis- and trans-acting factors required for the localization and translational control of these mRNAs are discussed along with potential mechanisms for their regulation.

The protein Mago provides a link between splicing and mRNA localization

The proteins Mago and Y14 are evolutionarily conserved binding partners. Y14 is a component of the exon-exon junction complex (EJC), deposited by the spliceosome upstream of messenger RNA (mRNA) exon-exon junctions. The EJC is implicated in post-splicing events such as mRNA nuclear export and nonsense-mediated mRNA decay. Drosophila Mago is essential for the localization of oskar mRNA to the posterior pole of the oocyte, but the functional role of Mago in other species is unknown. We show that Mago is a bona fide component of the EJC. Like Y14, Mago escorts spliced mRNAs to the cytoplasm, providing a direct functional link between splicing and the downstream process of mRNA localization. Mago/Y14 heterodimers are essential in cultured Drosophila cells. Taken together, these results suggest that, in addition to its specialized function in mRNA localization, Mago plays an essential role in other steps of mRNA metabolism.

The RNA-binding protein Tsunagi interacts with Mago Nashi to establish polarity and localize oskar mRNA during Drosophila oogenesis

In Drosophila melanogaster, formation of the axes and the primordial germ cells is regulated by interactions between the germ line-derived oocyte and the surrounding somatic follicle cells. This reciprocal signaling results in the asymmetric localization of mRNAs and proteins critical for these oogenic processes. Mago Nashi protein interprets the posterior follicle cell-to-oocyte signal to establish the major axes and to determine the fate of the primordial germ cells. Using the yeast two-hybrid system we have identified an RNA-binding protein, Tsunagi, that interacts with Mago Nashi protein. The proteins coimmunoprecipitate and colocalize, indicating that they form a complex in vivo. Immunolocalization reveals that Tsunagi protein is localized within the posterior oocyte cytoplasm during stages 1-5 and 8-9, and that this localization is dependent on wild-type mago nashi function. When tsunagi function is removed from the germ line, egg chambers develop in which the oocyte nucleus fails to migrate, oskar mRNA is not localized within the posterior pole, and dorsal-ventral pattern abnormalities are observed. These results show that a Mago Nashi-Tsunagi protein complex is required for interpreting the posterior follicle cell-to-oocyte signal to define the major body axes and to localize components necessary for determination of the primordial germ cells.

Drosophila Y14 shuttles to the posterior of the oocyte and is required for oskar mRNA transport

BACKGROUND

mRNA localization is a powerful and widely employed mechanism for generating cell asymmetry. In Drosophila, localization of mRNAs in the oocyte determines the axes of the future embryo. oskar mRNA localization at the posterior pole is essential and sufficient for the specification of the germline and the abdomen. Its posterior transport along the microtubules is mediated by Kinesin I and several proteins, such as Mago-nashi, which, together with oskar mRNA, form a posterior localization complex. It was recently shown that human Y14, a nuclear protein that associates with mRNAs upon splicing and shuttles to the cytoplasm, interacts with MAGOH, the human homolog of Mago-nashi.

RESULTS

Here, we show that Drosophila Y14 interacts with Mago-nashi in vivo. Immunohistochemistry reveals that Y14 is predominantly nuclear and colocalizes with oskar mRNA at the posterior pole. We show that, in y14 mutant oocytes, oskar mRNA localization to the posterior pole is specifically affected, while the cytoskeleton appears to be intact.

CONCLUSIONS

Our findings indicate that Y14 is part of the oskar mRNA localization complex and that the nuclear shuttling protein Y14 has a specific and direct role in oskar mRNA cytoplasmic localization.

Stable anterior anchoring of the oocyte nucleus is required to establish dorsoventral polarity of the Drosophila egg

In Drosophila, dorsoventral polarity is established by the asymmetric positioning of the oocyte nucleus. In egg chambers mutant for cap 'n' collar, the oocyte nucleus migrates correctly from a posterior to an anterior-dorsal position where it remains during stage 9 of oogenesis. However, at the end of stage 9, the nucleus leaves its anterior position and migrates towards the posterior pole. The mislocalisation of the nucleus is accompanied by changes in the microtubule network and a failure to maintain bicoid and oskar mRNAs at the anterior and posterior poles, respectively. gurken mRNA associates with the oocyte nucleus in cap 'n' collar mutants and initially the local secretion of Gurken protein activates the Drosophila EGF receptor in the overlying dorsal follicle cells. However, despite the presence of spatially correct Grk signalling during stage 9, eggs laid by cap 'n' collar females lack dorsoventral polarity. cap 'n' collar mutants, therefore, allow for the study of the influence of Grk signal duration on DV patterning in the follicular epithelium.

Barentsz is essential for the posterior localization of oskar mRNA and colocalizes with it to the posterior pole

The localization of Oskar at the posterior pole of the Drosophila oocyte induces the assembly of the pole plasm and therefore defines where the abdomen and germ cells form in the embryo. This localization is achieved by the targeting of oskar mRNA to the posterior and the localized activation of its translation. oskar mRNA seems likely to be actively transported along microtubules, since its localization requires both an intact microtubule cytoskeleton and the plus end-directed motor kinesin I, but nothing is known about how the RNA is coupled to the motor. Here, we describe barentsz, a novel gene required for the localization of oskar mRNA. In contrast to all other mutations that disrupt this process, barentsz-null mutants completely block the posterior localization of oskar mRNA without affecting bicoid and gurken mRNA localization, the organization of the microtubules, or subsequent steps in pole plasm assembly. Surprisingly, most mutant embryos still form an abdomen, indicating that oskar mRNA localization is partially redundant with the translational control. Barentsz protein colocalizes to the posterior with oskar mRNA, and this localization is oskar mRNA dependent. Thus, Barentsz is essential for the posterior localization of oskar mRNA and behaves as a specific component of the oskar RNA transport complex.

Molecular characterization and expression analysis of a highly conserved rice mago nashil homolog

Mago Nashi, a protein initially shown to be essential in the development of the Drosophila oocyte, is highly conserved among species and shows no homology to any other known cellular proteins. Here we report the nucleotide sequence of a cDNA and a partial gene that encode rice Mago Nashi protein homologs. In addition, we present the tissue-specific expression pattern of mago nashi at the level of RNA and protein. The rice Mago Nashi protein shares at least 73% amino acid identity with all known animal homologs. Genomic DNA gel blot analysis indicates that two copies of the mago nashi gene exist in the rice genome, one of which has identical intron positions to those found in an Arabidopsis homolog. mago nashi is expressed in root, leaf and developing seed tissue as determined by RNA and protein gel blot analysis. Evidence from Drosophila, Caenorhabditis elegans and human studies of Mago Nashi suggests that a major function of this protein is its involvement in RNA localization. The highly conserved amino acid sequence of all Mago Nashi protein homologs across kingdoms suggests that the plant version of this protein may similarly be involved in RNA localization.

Assembly of the Drosophila germ plasm

The Drosophila melanogaster germ plasm has become the paradigm for understanding both the assembly of a specific cytoplasmic localization during oogenesis and its function. The posterior ooplasm is necessary and sufficient for the induction of germ cells. For its assembly, localization of gurken mRNA and its translation at the posterior pole of early oogenic stages is essential for establishing the posterior pole of the oocyte. Subsequently, oskar mRNA becomes localized to the posterior pole where its translation leads to the assembly of a functional germ plasm. Many gene products are required for producing the posterior polar plasm, but only oskar, tudor, valois, germcell-less and some noncoding RNAs are required for germ cell formation. A key feature of germ cell formation is the precocious segregation of germ cells, which isolates the primordial germ cells from mRNA turnover, new transcription, and continued cell division. nanos is critical for maintaining the transcription quiescent state and it is required to prevent transcription of Sex-lethal in pole cells. In spite of the large body of information about the formation and function of the Drosophila germ plasm, we still do not know what specifically is required to cause the pole cells to be germ cells. A series of unanswered problems is discussed in this chapter.

RNA localization and translational regulation during axis specification in the Drosophila oocyte

The major axes of the oocyte-antero-posterior and dorso-ventral-are established over a one-day period during mid-oogenesis in Drosophila. The same molecule, GURKEN (GRK), functions to initiate signaling between the oocyte and the surrounding, somatically derived follicle cells. This results first in specification of the antero-posterior axis and, later, the dorso-ventral axis of the oocyte and surrounding follicle cells. Central to specification of both axes is a combination of cytoplasmic localization and translational regulation of the grk RNA. Here we discuss the mechanisms by which the grk RNA is localized within the oocyte and the role of translational regulation in spatially restricting the production of GRK protein. We then discuss the generality of these mechanisms during oogenesis by focusing on a second transcript, oskar, whose function is also regulated through a combination of transcript localization and translational control.

Merlin, the Drosophila homologue of neurofibromatosis-2, is specifically required in posterior follicle cells for axis formation in the oocyte

In Drosophila, the formation of the embryonic axes is initiated by Gurken, a transforming growth factor alpha signal from the oocyte to the posterior follicle cells, and an unknown polarising signal back to the oocyte. We report that Drosophila Merlin is specifically required only within the posterior follicle cells to initiate axis formation. Merlin mutants show defects in nuclear migration and mRNA localisation in the oocyte. Merlin is not required to specify posterior follicle cell identity in response to the Gurken signal from the oocyte, but is required for the unknown polarising signal back to the oocyte. Merlin is also required non-autonomously, only in follicle cells that have received the Gurken signal, to maintain cell polarity and limit proliferation, but is not required in embryos and larvae. These results are consistent with the fact that human Merlin is encoded by the gene for the tumour suppressor neurofibromatosis-2 and is a member of the Ezrin-Radixin-Moesin family of proteins that link actin to transmembrane proteins. We propose that Merlin acts in response to the Gurken signal by apically targeting the signal that initiates axis specification in the oocyte.

Mechanisms of RNA localization and translational regulation

Transcript localization and translational regulation are two post-transcriptional mechanisms for the spatial and temporal regulation of protein production. During the past year, two transcript localization mechanisms have been elaborated in some detail. Where localization involves directional transport on cytoskeletal tracks, links between the transcripts and the cytoskeletal molecular motors have been elaborated. In the case of localization by generalized transcript degradation combined with localized protection, trans-acting pathways and cis-acting elements for degradation and protection have been identified. A third transcript localization mechanism, vectorial transport out of the nucleus into a particular cytoplasmic domain, was initially thought to localize pair-rule transcripts in Drosophila. However, these have now been shown to be localized by directional transport in the cytoplasm. Transcript localization and translational regulation can be intimately linked in that, for certain messenger RNAs, only the localized fraction of transcripts is translated whereas unlocalized transcripts are translationally repressed. Cis-acting sequences and trans-acting factors that function in translational repression have been identified along with factors involved in relief of translational repression at the site of localization.

Combined activities of Gurken and decapentaplegic specify dorsal chorion structures of the Drosophila egg

During Drosophila oogenesis Gurken, associated with the oocyte nucleus, activates the Drosophila EGF receptor in the follicular epithelium. Gurken first specifies posterior follicle cells, which in turn signal back to the oocyte to induce the migration of the oocyte nucleus from a posterior to an anterior-dorsal position. Here, Gurken signals again to specify dorsal follicle cells, which give rise to dorsal chorion structures including the dorsal appendages. If Gurken signaling is delayed and starts after stage 6 of oogenesis the nucleus remains at the posterior pole of the oocyte. Eggs develop with a posterior ring of dorsal appendage material that is produced by main-body follicle cells expressing the gene Broad-Complex. They encircle terminal follicle cells expressing variable amounts of the TGFbeta homologue, decapentaplegic. By ectopically expressing decapentaplegic and clonal analysis with Mothers against dpp we show that Decapentaplegic signaling is required for Broad-Complex expression. Thus, the specification and positioning of dorsal appendages along the anterior-posterior axis depends on the intersection of both Gurken and Decapentaplegic signaling. This intersection also induces rhomboid expression and thereby initiates the positive feedback loop of EGF receptor activation, which positions the dorsal appendages along the dorsal-ventral egg axis.

mag-1, a homolog of Drosophila mago nashi, regulates hermaphrodite germ-line sex determination in Caenorhabditis elegans

The Caenorhabditis elegans gene mag-1 can substitute functionally for its homolog mago nashi in Drosophila and is predicted to encode a protein that exhibits 80% identity and 88% similarity to Mago nashi (P. A. Newmark et al., 1997, Development 120, 3197-3207). We have used RNA-mediated interference (RNAi) to analyze the phenotypic consequences of impairing mag-1 function in C. elegans. We show here that mag-1(RNAi) causes masculinization of the germ line (Mog phenotype) in RNA-injected hermaphrodites, suggesting that mag-1 is involved in hermaphrodite germ-line sex determination. Epistasis analysis shows that ectopic sperm production caused by mag-1(RNAi) is prevented by loss-of-function (lf) mutations in fog-2, gld-1, fem-1, fem-2, fem-3, and fog-1, all of which cause germ-line feminization in XX hermaphrodites, but not by a her-1(lf) mutation which causes germ-line feminization only in XO males. These results suggest that mag-1 interacts with the fog, fem, and gld genes and acts independently of her-1. We propose that mag-1 normally allows oogenesis by inhibiting function of one or more of these masculinizing genes, which act during the fourth larval stage to promote transient sperm production in the hermaphrodite germ line. When the Mog phenotype is suppressed by a fog-2(lf) mutation, mag-1(RNAi) also causes lethality in the progeny embryos of RNA-injected, mated hermaphrodites, suggesting an essential role for mag-1 during embryogenesis. The defective embryos arrest during morphogenesis with an apparent elongation defect. The distribution pattern of a JAM-1::GFP reporter, which is localized to boundaries of hypodermal cells, shows that hypodermis is disorganized in these embryos. The temporal expression pattern of the mag-1 gene prior to and during morphogenesis appears to be consistent with an essential role of mag-1 in embryonic hypodermal organization and elongation.

Regulation of the vitellogenin receptor during Drosophila melanogaster oogenesis

In many insects, development of the oocyte arrests temporarily just before vitellogenesis, the period when vitellogenins (yolk proteins) accumulate in the oocyte. Following hormonal and environmental cues, development of the oocyte resumes, and endocytosis of vitellogenins begins. An essential component of yolk uptake is the vitellogenin receptor. In this report, we describe the ovarian expression pattern and subcellular localization of the mRNA and protein encoded by the Drosophila melanogaster vitellogenin receptor gene yolkless (yl). yl RNA and protein are both expressed very early during the development of the oocyte, long before vitellogenesis begins. RNA in situ hybridization and lacZ reporter analyses show that yl RNA is synthesized by the germ line nurse cells and then transported to the oocyte. Yl protein is evenly distributed throughout the oocyte during the previtellogenic stages of oogenesis, demonstrating that the failure to take up yolk in these early stage oocyte is not due to the absence of the receptor. The transition to the vitellogenic stages is marked by the accumulation of yolk via clathrin-coated vesicles. After this transition, yolk protein receptor levels increase markedly at the cortex of the egg. Consistent with its role in yolk uptake, immunogold labeling of the receptor reveals Yl in endocytic structures at the cortex of wild-type vitellogenic oocytes. In addition, shortly after the inception of yolk uptake, we find multivesicular bodies where the yolk and receptor are distinctly partitioned. By the end of vitellogenesis, the receptor localizes predominantly to the cortex of the oocyte. However, during oogenesis in yl mutants that express full-length protein yet fail to incorporate yolk proteins, the receptor remains evenly distributed throughout the oocyte.

Spire contains actin binding domains and is related to ascidian posterior end mark-5

Spire is a maternal effect locus that affects both the dorsal-ventral and anterior-posterior axes of the Drosophila egg and embryo. It is required for localization of determinants within the developing oocyte to the posterior pole and to the dorsal anterior corner. During mid-oogenesis, spire mutants display premature microtubule-dependent cytoplasmic streaming, a phenotype that can be mimicked by pharmacological disruption of the actin cytoskeleton with cytochalasin D. Spire has been cloned by transposon tagging and is related to posterior end mark-5, a gene from sea squirts that encodes a posteriorly localized mRNA. Spire mRNA is not, however, localized to the posterior pole. SPIRE also contains two domains with similarity to the actin monomer-binding WH2 domain, and we demonstrate that SPIRE binds to actin in the interaction trap system and in vitro. In addition, SPIRE interacts with the rho family GTPases RHOA, RAC1 and CDC42 in the interaction trap system. Thus, our evidence supports the model that SPIRE links rho family signaling to the actin cytoskeleton.

RNA sorting in Drosophila oocytes and embryos

Many RNAs involved in determination of the oocyte, specification of embryonic axes, and establishment of germ cells in Drosophila are localized asymmetrically within the developing egg or syncytial embryo. Here I review the current state of knowledge about the cis-acting sequences involved in RNA targeting, RNA binding proteins; gene activities implicated in localizing specific RNAs, and the role of the tubulin and actin cytoskeletons in RNA sorting within the oocyte. Targeted RNAs are often under complex translational control, and the translational control of two RNAs that localize to the posterior of the oocyte, oskar and nanos, is also discussed. Prospects for filling gaps in our knowledge about the mechanisms of localizing RNAs and the importance of RNA sorting in regulating gene expression are also explored.

Versatility in signalling: multiple responses to EGF receptor activation during Drosophila oogenesis

The Drosophila epidermal growth factor receptor (EGFR) is active in different tissues and is involved in diverse processes such as patterning of the embryonic ectoderm, growth and differentiation of imaginal discs and cell survival. During oogenesis, the EGFR is expressed in the somatic follicle cells that surround individual oocyte-nurse cell complexes. In response to germline signals, the follicle cells differentiate in a complex pattern, which in turn leads to the establishment of the egg axes. Two recent reports have shown that the strategies used to pattern posterior follicle cells are different from those used to pattern dorsal follicle cells. In posterior follicle cells, EGFR activity is translated into an on-off response, whereas, in dorsal follicle cells, patterning mechanisms are initiated and refined by feedback that modulates receptor activity over time.

Germ plasm in Caenorhabditis elegans, Drosophila and Xenopus

Special cytoplasm, called germ plasm, that is essential for the differentiation of germ cells is localized in a particular region of Caenorhabditis elegans, Drosophila and Xenopus eggs. The mode of founder cell formation of germline, the origin and behavior of the germline granules, and the molecules localized in germline cells are compared in these organisms. The common characteristics of the organisms are mainly as follows. First, the founder cells of germline are established before the initiation of gastrulation. Second, the germline granules or their derivatives are always present in germline cells or germ cells throughout the life cycle in embryos, larvae, and adults. Lastly, among the proteins localized in the germ plasm, only Vasa protein or its homolog is detected in the germline cells or germ cells throughout the life cycle. As the protein of vasa homolog has been reported to be also localized in the germline-specific structure or nuage in some of the organisms without the germ plasm, the possibility that the mechanism for differentiation of primordial germ cells is basically common in all organisms with or without the germ plasm is discussed.

maelstrom is required for an early step in the establishment of Drosophila oocyte polarity: posterior localization of grk mRNA

We describe a mutant, maelstrom, that disrupts a previously unobserved step in mRNA localization within the early oocyte, distinct from nurse-cell-to-oocyte RNA transport. Mutations in maelstrom disturb the localization of mRNAs for Gurken (a ligand for the Drosophila Egf receptor), Oskar and Bicoid at the posterior of the developing (stage 3–6) oocyte. maelstrom mutants display phenotypes detected in gurken loss-of-function mutants: posterior follicle cells with anterior cell fates, bicoid mRNA localization at both poles of the stage 8 oocyte and ventralization of the eggshell. These data are consistent with the suggestion that early posterior localization of gurken mRNA is essential for activation of the Egf receptor pathway in posterior follicle cells. Posterior localization of mRNA in stage 3–6 oocytes could therefore be one of the earliest known steps in the establishment of oocyte polarity. The maelstrom gene encodes a novel protein that has a punctate distribution in the cytoplasm of the nurse cells and the oocyte until the protein disappears in stage 7 of oogenesis.